Identifying tripartite relationship among cortical thickness, neuroticism, and mood and anxiety disorders

The number of young adults seeking help for emotional distress, subsyndromal-syndromal mood/anxiety symptoms, including those associated with neuroticism, is rising and can be an early manifestation of mood/anxiety disorders. Identification of gray matter (GM) thickness alterations and their relationship with neuroticism and mood/anxiety symptoms can aid in earlier diagnosis and prevention of risk for future mood and anxiety disorders. In a transdiagnostic sample of young adults (n = 252;177 females; age 21.7 ± 2), Hypothesis (H) 1:regularized regression followed by multiple regression examined relationships among GM cortical thickness and clinician-rated depression, anxiety, and mania/hypomania; H2:the neuroticism factor and its subfactors as measured by NEO Personality Inventory (NEO-PI-R) were tested as mediators. Analyses revealed positive relationships between left parsopercularis thickness and depression (B = 4.87, p = 0.002), anxiety (B = 4.68, p = 0.002), mania/hypomania (B = 6.08, p ≤ 0.001); negative relationships between left inferior temporal gyrus (ITG) thickness and depression (B = − 5.64, p ≤ 0.001), anxiety (B = − 6.77, p ≤ 0.001), mania/hypomania (B = − 6.47, p ≤ 0.001); and positive relationships between left isthmus cingulate thickness (B = 2.84, p = 0.011), and anxiety. NEO anger/hostility mediated the relationship between left ITG thickness and mania/hypomania; NEO vulnerability mediated the relationship between left ITG thickness and depression. Examining the interrelationships among cortical thickness, neuroticism and mood and anxiety symptoms enriches the potential for identifying markers conferring risk for mood and anxiety disorders and can provide targets for personalized intervention strategies for these disorders.


Statistical analysis
1. To test the assumptions of linear regression Shapiro-Wilk normality tests were performed for depression, anxiety, and mania/hypomania residual score distributions.2. To test Hypothesis 1, and given a large number of independent variables, a separate penalized regression elastic-net model was used for variable selection, using the GLMNET (4.1) package 92,93 in R (version 4.0.3)for GM thickness.Elastic-net is a modified form of least squares regression that penalizes complex models with a regularization parameter (λ) and is sensitive to correlated variables 94,95 .The regularization parameter shrinks coefficients toward zero and eliminates unimportant terms 92 .tenfold cross-validation with an elastic net alpha = 0.5 identified the optimal penalty term (λ) that minimized mean cross-validated error, reduced the chances of overfitting, and enforced recommended sparsity in the solution 94 .λ of minimum mean crossvalidated error was selected to identify the variables in a model with the least mean cross-validated error.3. Multiple regression models (one each for depression, anxiety, and mania/hypomania as the dependent variable) in SPSS (version 27) were used to quantify effect sizes and the extent to which each identified variable from step 2, along with age, sex, and IQ, were associated with depression, anxiety, and mania/hypomania at a false discovery rate (FDR; q ≤ 0.05) 96 , to account for multiple comparisons.Given the high correlation among depression, anxiety, and mania/hypomania scores (Supplemental section, Table 1 all r > 0.7) an additional analysis using a composite pathology score (rounded ratio) was performed to assess relationships among GM thickness and all three clinical measures.The composite pathology score was calculated as the sum of the three weighted scores X 100 and rounded to the nearest integer.4. To test Hypothesis 2 mediation, the PROCESS macro bootstrapping algorithm was used 68 .Separate mediation models were tested for each significant GM variable (step 3) as independent variables (IVs) and depression, anxiety, and mania/hypomania scores as dependent variables (DVs).In each model, Neuroticism and its subfactors were operationalized as mediators to examine their potential influence on the statistically significant relationships identified between cortical thickness variables (step 3) and the severity of mood and anxiety symptoms 67 .False discovery rate correction (FDR; q ≤ 0.05) was used to account for multiple mediation tests.Bootstrapping (5000 iterations) was applied to calculate confidence intervals for mediating effects.To confirm the direction of the identified relationships, we also tested the alternative pathway with Neuroticism as IV, depression, anxiety, and mania/hypomania scores as DVs, and each GM variable as mediators.5. Sensitivity analysis.Participants having current anxiety or depressive diagnoses were excluded, and regression analysis, with the appropriate family (see results 1 below), was performed on the remaining sample (n = 169).

Results
1.The residual scores of depression, anxiety, and mania/hypomania did not meet regression assumptions (Shapiro-Wilk, p < 0.001), showed a positive skew, and had standard deviations larger than the means.We therefore used negative binomial models for all analyses 97 .2. Results of Hypothesis 1 from the elastic-net-with-cross-validation models identified the non-zero variables listed in the Supplemental section, Table 2.
Step 3 below shows the statistically significant results.3. Regression analysis.
Step I. To test Hypothesis 2 mediation analysis for each of the seven significant relationships from Analysis 3 was performed.Neuroticism was thus used as a mediator in seven mediation models for the following relationships: depression and left inferior temporal thickness, left pars opercularis thickness; mania/ hypomania and left inferior temporal thickness, left pars opercularis thickness; anxiety and left inferior temporal thickness, left pars opercularis thickness, and left isthmus cingulate thickness.The total effect reflects the relationship between GM and symptoms before the inclusion of the mediator; the direct effect reflects the relationship between GM and symptoms after the inclusion of the mediator; the indirect effect reflects the mediation effect of Neuroticism on the relationship between GM and symptoms.The "a" path reflects the relationship between GM and Neuroticism; the "b" path reflects the relationship between Neuroticism and symptoms.For detailed statistical results, see Table 3.

Discussion
Given that the role of GM cortical thickness in psychopathology remains unclear, we aimed to identify neural markers, associated with mood and anxiety symptoms in a large sample of young adults recruited across a range of subsyndromal to syndromal levels of these symptoms.Additionally, we explored the extent to which neuroticism mediated these relationships.We demonstrated common and distinct patterns of relationships between GM thickness in the left inferior temporal gyrus, the left pars opercularis, and the left isthmus cingulate, in relation to subsyndromal-syndromal symptoms.Specifically, we showed that cortical thickness in the left inferior temporal www.nature.com/scientificreports/gyrus was negatively related to depression, anxiety, mania/hypomania, and composite pathology score, while cortical thickness in the left pars opercularis was positively related to these measures.Thickness in the left isthmus cingulate cortex was positively related to anxiety.The results also indicate that some of these relationships were mediated by neuroticism, where specific neuroticism subfactors explained the significant relationships between left inferior temporal gyrus cortical thickness and both mania/hypomania and depression.Specifically, the NEO subfactor anger/hostility mediated the negative relationship between left inferior temporal gyrus thickness and mania/hypomania; while the NEO subfactor vulnerability to stress mediated the relationship between left inferior temporal gyrus thickness and depression.The specificity of the directions of these pathways were supported by showing that cortical thickness measures did not mediate relationships between the above Neuroticism subfactors and mania/hypomania and depression.To our knowledge, this is the first study to examine tripartite relationships among cortical thickness, neuroticism subfactors, and mood and anxiety disorder risk.Partially consistent with our first hypothesis, we observed negative relationships between left inferior temporal cortical thickness and the severity of anxiety, depression, mania/hypomania symptoms, aligning with previous studies that reported reduced inferior temporal gyrus thickness in adults with mood disorders 24,29,30,35,36 , and childhood abuse exposure and severity in adolescents 98 .Additionally, our composite pathology score supported this negative relationship with left inferior temporal cortical thickness.This region is known as a key part of the visual pathway implicated in object, face, and scene perception 99 .Functional connectivity studies have shown that this region is related to text-based memory 100 , and atrophy in this part of cortex is related to semantic dementia 101 .Taken together, these findings may point to cognitive deficits observed in both mood and anxiety [102][103][104][105] , underscoring the transdiagnostic relevance of this region in the susceptibility to mood and anxiety disorders.
We observed positive relationships between left pars opercularis thickness and severity of anxiety, depression, and mania/hypomania symptoms.Moreover, our analysis using the composite pathology score showed this same positive relationship with pars left pars opercularis, a part of ventrolateral prefrontal cortex, is involved in language production and comprehension 106 , and supports attention to unexpected stimuli [107][108][109][110] .These findings only partially support our hypotheses and parallel previous reports of a greater GM thickness in left pars opercularis in adults with depression 34,48 ; however, other studies have shown lower cortical thickness in vlPFC in adults with BD 31,36,37 and related to the number of (hypo)manic episodes 111 .Given that the vlPFC supports attention to unexpected and especially salient (e.g., negative emotional) stimuli [107][108][109][110] , and voluntary emotion regulation 112 , greater cortical thickness in this region might underlie greater attention to unexpected and negative emotional events in at-risk young adults.The combination of reduced inferior temporal gyrus thickness and associated cognitive deficits, and greater vlPFC cortical thickness associated with greater attention to unexpected and negative events, might thus result in greater mood and anxiety symptom severity.
Additionally, we observed anxiety-specific cortical thickening in the left isthmus cingulate cortex.This region supports internally directed thought [113][114][115] , and is involved in imagination, formation and consolidation of episodic memory [116][117][118][119] , and self-relevance assessment 120 .Given that we have not found an association between cortical thickness in this region and depression or mania/hypomania symptoms, perhaps thickening of the left isthmus cingulate cortex may suggest a link between internal thoughts and self-related memories and assessments, and development of anxiety symptoms.Thickness in this region may also help to differentiate the risk for anxiety disorders from the risk for mood disorders.
In support of our second hypothesis, relationships between GM cortical thickness and mood and anxiety symptoms severity were mediated by neuroticism, and by two neuroticism subfactors in particular-anger/hostility, and vulnerability.The anger/hostility subfactor mediated the relationship between left inferior temporal gyrus cortical thickness and mania/hypomania, where the relationship between lower inferior temporal gyrus cortical thickness and greater mania/hypomania severity was explained by higher levels of anger/hostility.Greater levels of anger have been reported in individuals with BD [121][122][123] .Our findings might thus reflect a neural mechanism linking neuroticism-related anger and predisposition to mania/hypomania.In addition, the vulnerability to stress subfactor mediated the relationship between left inferior temporal gyrus thickness and depression, where ("a" path-the relationship between GM and neuroticism subfactor, "b" path-the relationship between neuroticism subfactor and symptoms, c-the total effect of the relationship between GM and symptoms, c'-the direct effect of the relationship between GM and symptoms, i-the indirect effect, CL-Bootstrap confidence intervals).
the relationship between lower inferior temporal gyrus cortical thickness and greater depression was explained by greater vulnerability, in support of the established role of stress in depression [124][125][126] .These mediation findings together suggest that neuroticism along with decreased left inferior temporal cortical thickness plays an important role in young adults at risk to mood, but not anxiety disorders.The fact that the mediation effects were shown to the vulnerability and not the depression subfactor of neuroticism might reflect differences in the nature of the HRSD and NEO PI-R depression subfactor as discussed above.Taken together, these findings may contribute to earlier diagnosis and treatment of mood disorders.
A potential limitation of the study was the heterogenous sample, with some participants having anxiety and depressive disorders.Sensitivity analyses showed, however, that in participants without depressive or anxiety disorders, our principal findings remained consistent: a negative relationship was observed between left inferior temporal thickness and all three measures of symptom severity, while a positive relationship was identified with left pars opercularis thickness.Although our study included a large sample of young adults, replication of our findings in future studies is needed.Longitudinal studies can provide more precise inferences about the potential directionality and consistency of these relationships over time.It will also be beneficial to test these relationships in other age groups along with sex differences.
To our knowledge, the present study of a large sample of unmedicated young adults recruited across a broad range of subsyndromal to syndromal mood and anxiety symptoms measured with clinically valid and sensitive assessments, is the first to explore tripartite relationships among cortical GM thickness, neuroticism and its subfactors, and mood and anxiety symptoms severity.Given that neuroticism is closely related to mood and anxiety symptoms, examining the interrelationships among cortical thickness, neuroticism and mood and anxiety symptoms enriches the potential for identifying markers conferring risk for mood and anxiety disorders and targets for personalized intervention strategies for these disorders.

Figure 1 .
Figure 1.Scatterplots showing the relationships between GM thickness and mood and anxiety symptoms severity.(a) Predicted Hamilton Depression Scale and left inferior temporal thickness; (b) Predicted Young Mania Rating Scale and left inferior temporal thickness; (c) Predicted Hamilton Anxiety Scale and left inferior temporal thickness and left inferior temporal thickness; (d) Predicted Depression Scale and left pars opercularis thickness, (e) Predicted Young Mania Rating Scale and left pars opercularis thickness; (f) Predicted Hamilton Anxiety Scale and left pars opercularis thickness; (g) Predicted Hamilton Anxiety Scale and left isthmus cingulate thickness.Dashed lines represent 95% confidence intervals.

Figure 2 .
Figure 2. (a) Neuroticism subfactor N6 (vulnerabily) mediates the relationship between Left inferior temporal thickness and severity of depression, (b) Neuroticism subfactor N2 (anger/hostility) mediates the relationship between Left inferior temporal thickness and severity of mania/hypomania.X-GM thickness, M-mediator, Y-symptoms severity.("a"path-the relationship between GM and neuroticism subfactor, "b" path-the relationship between neuroticism subfactor and symptoms, c-the total effect of the relationship between GM and symptoms, c'-the direct effect of the relationship between GM and symptoms, i-the indirect effect, CL-Bootstrap confidence intervals).

Table 2 .
Negative binomial regression analysis results of relationships between clinical outcome measures and predictors.
CI, Confidence Interval; L, left; R, right; GM, Gray matter; q, False Discovery Rate (FDR).Benjamini-Hochberg FDR corrected adjusted p-value < .05.Exp (B) = odds ratio or a 1 unit change in predictor variable is an Exp (B) increase in the dependent variable (depression score).Significant p-values in italics, FDR-corrected q in bold.